1. Field of the Invention
This invention relates to an electrostatic image forming apparatus usable in a copying machine, a printer, a plotter, a facsimile machine, etc. In particular, this invention relates to a control device for controlling an applied electric field in the image forming apparatus.
2. Description of the Related Art
A conventional electric field image forming apparatus is disclosed in U.S. Pat. No. 3,689,935. This conventional image forming apparatus has electrodes having a plurality of openings (hereinafter called "apertures"), wherein a voltage is applied to each of the electrodes based on image data. Toner particles are controlled by the voltage to pass through the apertures. Thus, images are formed on a support member using the toner particles that have passed through the apertures.
An aperture electrode of this conventional image forming apparatus comprises a plate made of an insulating material, a continuous reference electrode formed on one side of the plate, a plurality of mutually-insulated control electrodes formed on the other side of the plate, the aperture electrode member having at least one row of apertures formed by extending through the above three components. This conventional image forming apparatus further comprises means for selectively applying electric potentials between the reference electrode and each of the control electrodes, means for supplying or providing electrostatically charged toner particles so that flows of the charged toner particles passing through the respective apertures are modulated by the applied electric potentials, and means for positioning the support member in the path of the flow of the toner particles, such that the support member is movable relative to the aperture electrode member.
In this conventional image forming apparatus, the reference electrode of the aperture electrode member faces the toner carrier and the control electrodes face the back electrode. To supply negatively charged toner particles from the toner carrier to the support member, a first control voltage is applied to each "on" control electrode, and a second voltage is applied to each "off" control electrode. Thus, when the apparatus is turned on, the toner particles on the toner carrier roller are introduced into the support member side of the aperture electrode member through the apertures corresponding to the "on" control electrodes.
In this apparatus, the first or on control voltage is positive, while the second or off control voltage is zero. When the apparatus is turned on, a positive voltage is applied to each "on" control electrode, so that an electrostatic force directed from the toner carrier to each "on" control electrode is established. Further, the toner particles are introduced onto the support member by an electric field developed between the support member and the aperture electrode member to thereby form a toner image. In contrast, when the apparatus is turned off, a voltage of 0 V is applied to each "off" control electrode, so that no electrostatic force directed toward the aperture electrode member or away from the toner carrier is established.
However, some toner particles will be deposited on the aperture electrode member without traveling toward the support member, due to the positive voltage applied to each "on" control electrode. In this case, some of the apertures will become clogged with the toner particles, thereby deteriorating the quality of the recorded image. Further, since no electric field is directly applied by the "off" control electrodes to the toner particles in the vicinity of the "off" control electrodes, the image recording apparatus is not completely controlled.
It should be appreciated that the term "when the apparatus is turned off" means when one or more of the apertures are turned off, so that the toner particles are not being applied to the support member through the off apertures, that is, when a blank image portion is being formed. On the other hand, it should be appreciated that the term "when the apparatus is turned on" means when one or more of apertures are on, so that the toner particles are applied to the support member and an image is formed on the support member.